<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <script src="https://cdn.bootcss.com/vue/2.5.18-beta.0/vue.js"></script>

</head>
<body>
    <div id="app">
        {{sum}}
    </div>
    <script>
        var vm = new Vue({
            el:"#app",
            data:{
                num1:10,
                num2:20,
                sum:30
            },
            //监听data里面的某个值改变的时候触发
            watch:{
                num1(){
                    this.sum = this.num1+this.num2
                },
                num2:function(){
                    this.sum = this.num1+this.num2
                }
            }
        })
    </script>
</body>
</html>